1. Field of the Invention
The present invention relates generally to transgenic plants. More specifically, it relates to a methods and compositions expressing transgenes in plants.
2. Description of the Related Art
Recent advances in molecular biology have dramatically enhanced the ability of scientists to manipulate the germplasm of animals and plants. Genes controlling specific phenotypes, for example, particular polypeptides that lend insect, antibiotic and herbicide resistance, have been located within certain germplasm and isolated from it. Even more important has been the ability to take the genes which have been isolated from one organism and to introduce them into another organism. This transformation may be accomplished even where the recipient organism is from a different phylum, genus or species from that which donated the gene (heterologous transformation).
Attempts have been made to genetically engineer desired traits into plant genomes by introduction of exogenous genes using a number of genetic engineering techniques. The uptake of new DNA by recipient plant cells has been accomplished by means including Agrobacterium infection (Nester et al., 1984), polyethylene glycol (PEG)-mediated DNA uptake (Lorz et al., 1985), electroporation of protoplasts (Fromm et al., 1986) and microprojectile bombardment (Klein et al., 1987).
While some of the aforementioned techniques have made transformation of plants nearly routine, the expression of exogenous DNA has been more troublesome. One of the most serious problems which has been encountered is a phenomenon known as “co-suppression.” This term was coined to describe the inhibition of gene expression of an endogenous gene after the introduction of a homologous transgene (Jorgensen, 1990), and was first described for the chalcone synthase (CHS) gene in Petunia (Napoli et al., 1990; Van der Krol et al., 1990). Co-suppression is not unique to CHS, however, and appears to be a general phenomenon affecting transgenic plants. The degree of co-suppression varies for individual transformants, but in some plants, it may take place to such a degree that a null phenotype is produced for the loci involved.
Numerous transgenic plant systems have exhibited the phenomenon of homology-dependent “gene silencing,” which can involve either multiple copies of at least partially homologous transgenes or a transgene and a homologous endogenous sequence (Jorgensen, 1995; Matzke and Matzke, 1995; Meyer, 1995). The most fundamental mechanistic feature distinguishing various cases of silencing is whether the observed inactivation occurs at the transcriptional or post-transcriptional level, and this is determined in turn by the region of homology between the interacting sequences. Transcriptional silencing occurs largely as a result of promoter homology (Neuhuber et al., 1994).
Promoter homology-dependent gene silencing interferes with transcription, and sometimes causes paramutations, leading to heritable changes in gene expression and/or DNA modifications that persist after segregation of the transgene (Lindbo et al., 1993; Jorgensen, 1995; Matzke and Matzke, 1995; Park et al., 1996). The cause of such changes in gene expression are poorly understood, but it is known that silencing is influenced by the length of the homology and by the position of the interacting sequences.
In the case of the nopaline synthase promoter, it was found that a 300 bp region of homology was sufficient to mediate co-suppression in tobacco (Matzke et al., 1993). It has also been found that an endogenous sequence known as H2, which has homology to the nopaline synthase promoter, is a potent silencer of genes driven by the nopaline synthase promoter (Matzke et al., 1993; Matzke et al., 1994). This is believed to involve pairing of the nopaline synthase promoter copies at the silencing and target loci, followed by the imposition of methylation on the target copy to a degree similar to that acquired autonomously by the silencer (Matzke et al., 1994). The most efficient example of co-suppression is a tobacco line carrying a transgene insert with two genes driven by the 19S and 35S promoter of CaMV, respectively. Both genes linked to the two promoters are suppressed, and this locus trans-inactivates newly introduced constructs that provide at least 90 bp of common homology (Vaucheret, 1993).
Transcriptional silencing is particularly troublesome to agricultural biotechnologists, in that many of the most useful promoters for expression of a particular transgene are native to the host genome. This is especially true for one of agriculture's most important crops, maize. Examples of several maize promoters with desirable expression profiles include near constitutive maize promoters such as those of the Adh and sucrose synthase genes (Walker et al., 1987; Yang and Russell, 1990), tissue-specific promoters such as the maize zein and light harvesting complex promoters (Conkling et al., 1990; Simpson, 1986), and inducible promoters such as that of the corn heat shock protein (Odell et al., 1985).
There is, therefore, a great need in the art for improved methods for the expression of endogenous genes in plants, and particularly in agronomically important monocot plants such as maize. Particularly, methods are needed which allow scientists to exploit the desirable characteristics of monocot promoters, yet avoid the problems associated with co-suppression of homologous sequences. Currently technology is limited in this respect by the lack of suitable alternatives to promoters which are native to agronomically important monocot species.